It is known to use a single-mode fiber in a two-photon fluorescence endoscope. The fiber has a core with a small numerical aperture (NA), which acts as both the source and detection aperture. Transmission characteristics of the single-mode fiber allow for relatively high excitation efficiencies and resolution to be achieved but low collection efficiency results.
Multi-mode fibers, such as double-clad photonic crystal fibers (DCPCFs) have recently been used for imaging. Such fibers have a single-mode inner core, with a small NA to provide a high efficiency excitation signal. A larger NA multi-mode layer surrounds the core, within an outer cladding, to provide high collection efficiency, although somewhat reduced resolution as compared to the single-mode fiber.
The advantages of DCPCFs have been demonstrated for optical imaging in endoscopy, as disclosed in an article D. Yelin, et al, “Double-clad fiber for endoscopy”, Optics Letters, Vol. 29, No. 20, 2408 (2004). Further, improved detection efficiency has been investigated in relation to two-photon fluorescence detection, as disclosed in M. Myaing, et al, “Enhanced two-photon biosensing with double-clad photonic crystal fibers”, Optics Letters, Vol. 28, No. 14, 1224 (2003).